This invention relates generally to the treatment of molten metals, and more particularly to a method of and apparatus for purifying molten metals such as aluminum and aluminum alloys.
In one type of conventional continuous casting operation, aluminum or an alloy thereof is melted in a furnace and transferred via other receptacles to the pouring tundish of a continuous casting machine. Entrained within the molten aluminum are oxides of other metals existing as impurities, non-metallic oxides and other solid contaminants in particle form, such as, for example, finely divided particles of aluminum oxide broken off from the skin which forms on the surface of the molten aluminum. In addition, gaseous products, particularly hydrogen gas generated by reaction of the molten aluminum with moisture in the various receptacles of the system and in the surrounding environment, are commonly entrapped or dissolved in the molten aluminum. If not removed from the molten metal prior to the casting thereof, both the particulate matter and the gaseous products may contribute to undesirable voids and inclusions in the cast metal. When the cast metal is subsequently worked to produce a final product, such as continuously formed rod and wire, the inclusions and voids frequently cause flaws resulting in breaks and fissures in the worked product, poor mechanical properties and a reduction of the electrical conductivity of the final product.
The prior art proposals for overcoming the aforementioned difficulties include a number of methods of and apparatus for removing gaseous hydrogen from the molten metal by passing a fluxing gas therethrough. Many filtering systems and processes have also been proposed for removing the solid particulate impurities from the molten aluminum prior to the introduction of the metal into a casting mold. A number of well-known prior art fluxing and filtering systems employ a filter medium comprising a bed of loose refractory granules, such as, for example, aluminum oxide balls, through which the molten aluminum is passed in countercurrent relation to a fluxing gas. The aluminum treatment systems disclosed in U.S. Pat. No. 3,039,864 and U.S. Pat. No. 3,737,304 typify such prior art fluxing and filtering systems. With the prior art granular filter mediums, however, control of the particle size being filtered is not readily accomplished and the filter beds are often subjected to channeling problems which result in loss of filtering efficiency and frequent clogging. A clogged filter bed, of course, requires either a system shutdown to remove and replace the filter bed or, at least, an interruption of the casting operation to perform a cleaning of the filter bed in situ.
Other prior art filtering systems, such as that disclosed in U.S. Pat. No. 3,654,150, utilize a filter medium formed of one or more contiguous layers of a woven glass cloth fabric or screen through which the molten aluminum is filtered before it is introduced into the casting machine. Extreme caution must be taken, however, when using such glass cloth filters in order to prevent their rupture because of the deleterious effects on the strength of the glass fibers by the high temperature molten aluminum. In many instances, prior filtering systems using glass cloth filters are rendered ineffective by a rapid buildup of large quantities of particulate matter resulting in the clogging of the filter medium and the necessity to terminate operations to replace or renew the filter medium. Frequently, and especially as the filter becomes contaminated with filtered particulate matter, the downstream side of a glass cloth filter may be exposed to air because of the filter location or orientation or because of the flow rate at which the filtered metal is withdrawn. This results in the formation of aluminum oxide on the downstream side of the filter as well as an undesirably large pressure differential across the filter, the force of which could rupture the same.